Covert Communications for Text Semantic With Finite Blocklength

Semantic communication, by the extraction of essential semantic information from source data, reduces data volume and enhances transmission efficiency, making it increasingly popular among candidate technologies in 6G. In this letter, a covert transmission scheme for text semantic communications is proposed, where a transmitter attempts to send text semantic information to a legitimate receiver under the surveillance of a warden, while the receiver emits artificial noise (AN) to interfere the warden. To maximize semantic spectral efficiency, we formulate an optimization problem while considering constraints on covertness, the minimum semantic similarity, and the number of semantic symbols mapped per word K. We derive the closed-form expressions for the optimal transmit power and AN power when K is fixed, and employ a one-dimension searching method to find the optimal <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$K^{*}$ </tex-math></inline-formula>. Numerical results demonstrate that fixed AN power can contribute to covert transmission and in the semantic model, <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$K^{*}$ </tex-math></inline-formula> is the minimum allowable K.